Fuel efficiency is a controlling factor in the design of modern vehicles. Vehicle designers wage a constant battle with vehicle weight and performance in order to provide increased fuel efficiency. The desire for improved fuel efficiency, however, cannot come at the cost of expected vehicle performance. As such, in many environments the need for part time all wheel drive is accepted as a design necessity even though it commonly comes with a penalty to fuel efficiency. One purveyor of this inefficiency is the power transfer units commonly utilized in modern vehicle production.
Power transfer units are commonly utilized in front wheel drive based all wheel drive systems. A power transfer unit transmits the torque from the transmission to a propshaft, which in turn delivers power to the rear wheels. Most power transfer units are always in a ready state, commonly controlled by a slipping clutch near the rear axle, and yet are utilized only a small fraction of the time during driving. In this “ready state” the existing power transfer units exhibit a full-time drain to fuel efficiency with only a part-time benefit. A method and system that would allow in increase in efficiency more on par with the part-time benefit would be highly beneficial.
Thus, in front-wheel-drive based all-wheel-drive systems, there exists a need for improving the efficiency of power transfer units such that their full-time drain on fuel efficiency is negated or minimized.